A common form of metal detector is that in which the metal detector transmits a transmit magnetic field and receives a receive magnetic field radiated by elements of the environment that are influenced by that transmit magnetic field. Examples of this type of metal detector include detectors that are used for detecting explosive landmines, unexploded ordnance, precious native metals, coins, jewellery and caches of similar items, in ground. Development of metal detectors has involved not only the improvement of sensitivity to sought objects but, through processing signals due to the receive magnetic field radiated by them, the separation of their radiated signals from those radiated by objects in which the detectorist has less interest, as well as the classification of the nature of the sought target.
Transmission, reception of the receive magnetic field and processing of the signals due to the receive magnetic field are effected through electronic circuitry.
These electronic metal detectors usually include transmit electronics for generating a repeating transmit signal cycle of a fundamental period applied to an inductor, a transmit coil, that transmits a changing magnetic field often referred to as a transmit magnetic field.
Metal detectors can be divided into two main types, depending upon the nature of their transmit field; CW (Continuous Wave) and PI (Pulse Induction) detectors. Some CW detectors transmit magnetic fields that have substantially only one sinusoidal frequency. Others can transmit a transmit magnetic field that is a combination of some small, finite, number of frequencies or, through application of a rectangular transmit signal to a broadband transmit circuit, a large number of frequencies.
PI detectors usually include switching electronics in the transmit electronics, producing a large number of frequencies in the transmit field.
Another way of classifying metal detectors is to class them as either time-domain or frequency-domain detectors. This classification of detectors has more to do with how the receive signals are processed, rather than the nature of the transmit field. Frequency-domain metal detectors could use either PI or CW transmit systems, but usually use CW. Time-domain detectors are commonly seen with either CW or PI transmit systems, but the nature of the transmit magnetic field must be such that it contains a large number of frequencies.
In the types of metal detector described herein, the processing of receive signals is done through synchronous demodulation of an amplified signal induced in a receive coil. In frequency-domain detectors, each channel of demodulation is designed to be most responsive to a single frequency and phase of the receive signal. In time-domain detectors, each channel of synchronous demodulation is designed, by necessity, to be sensitive to a range of frequencies. In some time-domain detectors, there are several channels of synchronous demodulation, each channel emitting a signal representative of a different broad group of frequencies.
Metal detectors contain receive electronics which processes a receive signal from a measured induced voltage due to a receive magnetic field to produce an indicator signal, the indicator output at least indicating the presence of at least some metallic targets within the influence of the transmit magnetic field.
There is a problem with this, whenever the receive signal includes a signal due to the presence of one or more desired metallic objects, it is often mixed with, or contaminated by, unwanted signals, for example signals due to unwanted buried objects (such as ferrous objects), background noises, and signals due to soil or ground. Indeed, it is possible that a receive signal that is composed entirely of unwanted signals will be able to elicit the indication of a target, even when there is no desired target present.
In metal detectors that classify detected targets as well as merely detecting them, contaminated receive signals, if not further processed, can result in erroneous classification of the targets. This can further lead to reduced sensitivity to small or deeply-buried sought objects. Acceptable detection performance can be difficult to achieve without further processing of the receive signal.
A known technique for improving the processing of the receive signals and the reliability of detection is to estimate the unwanted signals prior to applying further processes to a receive signal, then using a function of that estimation to cancel the unwanted signals from the receive signal. This adjusted receive signal is then processed. However, estimation of the unwanted signals generally does not produce a reliable cancellation of the unwanted signals from the receive signal, resulting in reduced reliability of detection of a wanted target.